#!/bin/sh ionice -c 3 find /home/<sitename>/public_html/var/session/ -name 'sess*' -mtime +3 -type f -delete
Replacing the "<sitename>" value with you own location. This will lower the priority of the command in the O/S and also use the "find" commands own delete mechanism which all-in-all is much faster than passing it to rm.
This can then be added to CRON using
cron -e so that it executes on a regular basis:
# remove magento sessions 0 3 * * * /bin/sh /home/<sitename>/session_cleanup.sh